Detonators may be used ms explosive devices per se but are generally used to initiate other explosives. In general terms they have an input end for a triggering signal, customary an electric voltage or the heat and shock from a fuse, and an output end commonly containing a base charge of secondary explosive. Between the input and output ends, means are provided for securing a transformation of the input signal into a detonation of the base charge. In civilian detonators this is generally accomplished by the presence of a small amount of primary explosive adjacent the base charge, which primary explosive rapidly and reliably detonates when subjected to heat or shock. On the other hand, the high sensitivity of primary explosives calls for severe safety precautions in detonator manufacture and use. Primary explosives cannot be transported in bulk but has to be locally produced at each detonator plant. In addition to the high relative manufacturing costs in small units, most primary explosives entail handling of poisonous or hazardous substances. Within the plant the explosive has to be treated and transported in small batches and final dosage and pressing has to be performed by remotely operated devices behind blast shields. In the detonator product the presence of primary explosive is a potential cause of unintentional detonation during transport and use. Any damage, impact, heat or friction at the primary explosive site may trigger the detonator. The primary explosive may also pick up the shock from a neighboring detonation and cause mass detonation in closely arranged detonators. For these reasons strict vernmental regulations are placed on detonator transports. On-site handling are subjected to similar restrictions.
Efforts have been made to replace the primary explosives with the much less dangerous secondary explosives used for example in the base charges. A non-primary detonator should simplify manufacture, permit free transportation including transportation on aircrafts and reduce use restrictions, e.g. allowing concurrent drilling and charging operations.
Igniting devices of the exploding wire or exploding foil type, for example according to the French patent specification 2 242 899, are able to produce a shock of sufficient strength to directly induce detonation in secondary explosives when exposed to high momentary electic currents. They are normally not suitable in civilian applications since expensive and elaborate blasting machines are required and since they are compatible with ordinary pyrotechnical delay devices.
Another type of non-primary explosive detonators, as represented by U.S. Pat. Nos. 3,978,791, 4,144,814 and 4,239,004, suggests use of initiated and deflagrating secondary explosive for acceleration of an impactor disc to impinge on an acceptor secondary explosive with sufficient velocity to detonate the acceptor explosive. To withstand the forces involved the designs are large and mechanically complicated and not entirely reliable.
Still another type of non-primary explosive detonators, as represented by the U.S. Pat. No. 3,212,439, utilizes the ability of ignited and deflagrating secondary explosives to spontaneously transit form deflagration to detonation under suitable conditions. These conditions normally include heavy confinement of rather large amounts of the explosive, which adds to cost and size when compared to conventional primary explosive detonators.
Broadly, successful commercialization of these known types of non-primary explosive detonators have been restricted by by at least two circumstances. The first is the requirement for complex design or heavy confinement, which adds to both material and manufacturing cost when regular production equipments cannot be used. Out of standard size represents an additional cost also for the user. Secondly, while it is possible to obtain some function with various non-primary detonator designs, it is very difficult to reach the very high initiation reliability of primary explosive detonators. Such a high reliability is required by the customers in order to avoid the dangerous task of dealing with an undetonated borehole charge.
Improvements in the above aspects meet partially contradictory requirements. Reduced confinement may reduce also reliability in function or at least limits operational tolerances which adds to manufacturing rejection and control costs. A simple and small design of the detonator part where deflagration to detonation take place may require more elaborate igniting means to establish rapid and reproducible deflagration.
The U.S. Pat. No. 4,727,808 dicloses a new kind of non-primary explosive detonator based on a deflagration to detonation transision of a secondary explosive. The design described can be ignited by most kinds of conventional igniting means, can be manufactured by use of conventional detonator cap equipments, can be housed in normal detonator shells and can be reliably detonated with only slight confinement of the secondary explosive charge. Initiation reliability can be further improved, however, especially at extreme conditions.